Numerous semiconductor waveguide devices, such as optical loss and phase modulators, are used in association with laser diodes to perform a wide variety of applications in optical communication and signal processing systems. The waveguide devices may be made by epitaxially depositing on a substrate a series of thin layers of semiconductor such as III-V compound semiconductor materials of the appropriate compositions. Channel waveguides are formed in the layers by using standard processing techniques.
An example of an emerging technology where precise measurements are needed is in the processing of silicon substrates which may require back surface thinning of the substrate after micro-circuitry has been fabricated on the front surface. Since the thinning has been accomplished by means of an etching technique which must be controlled precisely, designers and fabricators need to be able to measure the substrate thickness during the etching process. Because of reliability and satisfactory operational considerations, this need becomes more acute so that the final thickness variations of the etched silicon substrate must be with an accuracy on the order of 0.1 microns.
In addition to knowing thickness measurements for the silicon substrate application it is important to know the optical property characterizations of waveguide devices which will affect their performance. Various methods have been used to measure the parameters of optical waveguides and waveguide modulators. All have been noted to have deficiencies and limitations and, furthermore, no single measurement system can be used to determine all of the relevant device parameters.
For example, attenuation measurements have been made by coupling an optical signal into and out of a waveguide using a pair of coupling prisms, see the article by F. Zernike et al. appearing in the Journal of Optical Society of America 61, p. 678 (1971). The optical power coupled out of the waveguide was measured at a particular position along the waveguide. The prism was then moved along the waveguide a certain length and the output power again was measured. By measuring the length change between the two measurement points, a value could be obtained for the attenuation of the waveguide. Discrepancies could arise due to changes which might occur in the output coupling, which is very sensitive to the positioning of the prism with respect to the waveguide. In addition, because of the relatively large size of the coupling prisms, this method is limited to rather long waveguides, those being on the order of centimeters. Another contemporary attenuation measurement method utilizes a detector probe which is scanned along the length of the waveguide to measure the scattered optical power as a function of length, see the article by J. E. Goell, appearing in the Proceedings of the IEEE 58, p. 1504 (1970). The recording of scattered power versus length is used to obtain a value for the waveguide attenuation. However, a certain degree of unreliability could creep in if the scattering in the waveguide is not uniform along its length. A third known method of measuring waveguide attenuation is more suited to short-length waveguides and uses a Fabry-Perot interferometer configuration, a highly coherent source, and temperature or electro-optic tunings to vary the index of refraction, see the article by Y. Matsui et al. appearing SPIE Vol. 651, p. 263 (1986). This measurement system is complex and requires a somewhat involved interpretation of the measurement results to yield an attenuation figure.
The measurements of the absolute effective index of semiconductor waveguides and changes in the index due to applied modulations and external disturbances appear to be not well developed. Changes in the index can be measured by placing the waveguide sample in a Mach-Zehnder interferometer configuration, applying the modulation or external disturbance, and counting interference fringes such as that described in the article by H. Soga et al., appearing in Electronics Letters (1988). The method of the article is limited to only measuring the change in index.
Thus, continuing need exists in the state of the art for a partial coherence interferometer used to demonstrate accurate measurements of the waveguide parameters of attenuation, effective index and changes in attenuation and effective index with applied modulation fields and external perturbations. Another application is to provide a precise measurement of thickness of substrates that lends itself to in-situ measurements during processing.